The invention relates to a method for shortening the cold starting time of a CO oxidation catalytic converter in vehicles with an integrated fuel cell.
After a vehicle with an integrated fuel cell system has been operating, H2O and fuel (for example methanol) condense out of the reformate gas of the gas-generating system on the surface (for example platinum) of the CO oxidation catalytic converter owing to the cooling down of the reformate gas. In addition, after the vehicle has been operating, the CO oxidation catalytic converter is coated with CO by chemisorption. As a result, the pores and active centers on the surface of the CO oxidation catalyst are coated. Thus, depending on the degree of coverage of the surface of the catalyst by these gas components, the ignition temperature level of the catalytic converter is raised, as a result of which the starting time of the catalytic converter is lengthened in the case of cold starting. In an extreme case, ignition may even be prevented. When water is present, icing up of the catalytic converter may even occur at low temperatures.
The object of the invention is therefore to provide a method which reliably decreases or eliminates coating of the pores and active centers of the surface of the catalyst by gas components which delay the start after the vehicle has been switched off.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which gas components that delay the start are removed from the reformate gas of the gas-generating system by means of an adsorber, during the cooling-down phase after the vehicle has been switched off. In particular, it is possible to adsorb condensing components such as water or fuel (for example, methanol and CO). Therefore, at the next cold start, the surface of the CO oxidation catalyst is thus free of such components which delay the start. As a result, the starting time of the CO oxidation catalytic converter can be shortened and the CO emissions decreased.
According to the invention, the gas components which delay the start are depleted out of the reformate gas by adsorption after the vehicle has been switched off. The gas components which are to be adsorbed out of the reformate gas are conveyed to the adsorber by means of, in particular, diffusion processes. In order to accelerate these conveying processes, it is possible to meter in secondary air upstream of the adsorber. Coating of the surface of the CO oxidation catalyst with H2O, fuel (for example methanol) and CO when the reformate gas cools down can thus be reduced. Effective adsorption of the gas components can be achieved in particular by means of the following properties of the adsorber and of the adsorber material:
suitable adsorption isotherms for the substances to be adsorbed. That is, the adsorber material has, for example a high selectivity and affinity with respect to H2O, for example a zeolite with hydrophilic properties (molecular sieve 13xc3x97).
increased contact area between the reformate gas and the adsorber. This achieves faster desorption kinetics (in particular during the operating phase) and faster adsorption kinetics (during the cooling-down phase after the vehicle has been switched off).
availability of a high adsorptivity of the adsorber material even at temperatures above the dew point of the reformate gas (typically 80-85xc2x0 C. for _(water)=1.3).
the adsorber material having a high degree of flowability in order to prevent agglomeration of the adsorber material at high coating levels.
the material having a high settled apparent density of the adsorber (diameter of the pellets used is less than 2 mm) and a high degree of porosity; and
a high mechanical stability of the adsorber material. In particular, particle fractions as a result of abrasion should be avoided.
In one embodiment according to the invention, the adsorber is arranged downstream of the CO oxidation catalytic converter in the fuel cell system. In this case, it is necessary to take into account the fact that a highly exothermic reaction occurs in the reformer at which the reformate gas is produced, and the reformate gas is thus strongly heated at this point. As a result, a temperature gradient forms from the reformer via the CO oxidation catalyst to the adsorber connected downstream. In the cooling-down phase after the vehicle has been switched off, the adsorber continues to be at a lower temperature level than the CO oxidation catalyst, as a result of which it is possible to load the adsorber to a greater degree with gas components which delay the start. In order to improve the temperature difference between the adsorber and CO oxidation catalyst further, the adsorber can be cooled either actively or passively.
However, in a further arrangement according to the invention it is possible, given appropriate cooling, also to arrange the adsorber upstream of the CO oxidation catalytic converter.
In order to achieve a high adsorption yield at the adsorber it is also possible for the reformer to be thermally insulated.
The adsorber can be desorbed during the operating phase of the vehicle. To do this, the adsorber is desorbed to the moisture level of the reformate gas. The desorption processes which occur here are predominantly displacement desorption (as a result of CO2, for example) and thermal desorption at temperatures of the reformate gas below 100xc2x0 C.
For faster and more effective desorption of the adsorber, it is possible according to the invention to meter in air, for example compressor air from an air metering point, upstream of the adsorber during the operating phase of the vehicle. However, the desorption can also take place after the vehicle has been switched off, preferably directly after the vehicle has been switched off, before the CO oxidation reactor has reached the dew point. Air can also be metered into the reformate gas for this purpose.
In one preferred embodiment, use is advantageously made of an adsorber which is bounded by an outer casing with a cylindrical pipe passing through it, in which the reformate gas flows. The space between the outer casing and the pipe is filled with the adsorber material. The part of the pipe which is located within the outer casing is perforated. In this context it is possible, for example, to use a mesh wire. The diameter of the perforations must be smaller here than the diameter of the adsorber material which is in the form of a bulk material.
An advantage of the described adsorber is that when it is used there is no pressure drop, or only a very small pressure drop in the reformate stream. In addition, the separation of the pipe and the adsorber material according to the invention minimizes mechanical abrasion of the adsorber material.
In one advantageous embodiment, the dwell time of the reformate gas within the adsorber can be prolonged by means of twists in the pipe. The exchange surface between the gas and adsorber material can also be increased by structuring the surface of the pipe. This results in a higher adsorption yield of the gas components from the reformate gas which delay the start.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.